ALGOL 68C

Below is a sampling of some notable extensions:

• Automatic op:= for any operator, e.g. *:= and +:=
• UPTO, DOWNTO and UNTIL in loop-clauses;
• displacement operator (:=:=)
• ANDF, ORF and THEF syntactic elements.
• separate compilation - ENVIRON clause and USING clause
• scopes not checked
• bounds in formal-declarers
• CODE ... EDOC clause - for embedding ZCODE

The ENVIRON and USING clauses

Separate compilation in ALGOL 68C is done using the ENVIRON and USING clauses. The ENVIRON saves the complete environment at the point it appears. A separate module written starting with a USING clause is effectively inserted into the first module at the point the ENVIRON clause appears.

ENVIRON and USING are useful for a top-down style of programming, in contrast to the bottom-up style implied by traditional library mechanisms.

These clauses are kind of the inverse of the #include found in the C programming language, or import found in Python. The purpose of the ENVIRON mechanism is to allow a program source to be broken into manageable sized pieces. It is only necessary to parse the shared source file once, unlike a #include found in the C programming language where the include file needs to be parsed for each source file that includes it.

Example of ENVIRON clause

A file called mylib.a68:

BEGIN INT dim = 3; # a constant # INT a number := 120; # a variable # ENVIRON EXAMPLE1; MODE MATRIX = [dim, dim]REAL; # a type definition # MATRIX m1; a number := ENVIRON EXAMPLE2; print((a number)) END

Example of USING clause

A file called usemylib.a68:

USING EXAMPLE2 FROM "mylib" BEGIN MATRIX m2; # example only # print((a number)); # declared in mylib.a68 # print((2 UPB m1)); # also declared in mylib.a68 # ENVIRON EXAMPLE3; # ENVIRONs can be nested # 666 END

• No ALGOL 68 FLEX and variable length arrays
• MODE STRING implemented without FLEX
• The PAR parallel clause was not implemented
• Nonstandard transput
• others...

A translator-compiler for ALGOL 68C was available for the PDP-10, IBM System/360 and several other computers.

A very early predecessor of this compiler was used by Guy and Bourne to write the first Game of Life programs on the PDP-7 with a DEC 340 display.

Various Liverpool Software Gazette issues detail the Z80 implementation. The compiler required about 120 KB of memory to run; hence the Z80's 64 KB memory is actually too small to run the compiler. So ALGOL 68C programs for the Z80 had to be cross-compiled from the larger CAP computer, or an IBM System/370 mainframe computer.

Stephen Bourne subsequently reused ALGOL 68's if ~ then ~ else ~ fi, case ~ in ~ out ~ esac and for ~ while ~ do ~ od clauses in the common Unix Bourne shell, but with in's syntax changed, out removed, and od replaced with done (to avoid conflict with the od utility).

After Cambridge, Bourne spent nine years at Bell Labs with the Version 7 Unix (Seventh Edition Unix) team. As well as developing the Bourne shell, he ported ALGOL 68C to Unix on the DEC PDP-11-45 and included a special option in his Unix debugger Advanced Debugger (adb) to obtain a stack backtrace for programs written in ALGOL 68C. Here is an extract from the Unix 7th edition manual pages:

NAME adb - debugger SYNOPSIS adb [-w] [ objfil [ corfil ] ] [...] COMMANDS [...] $modifier Miscellaneous commands. The available modifiers are: [...] a ALGOL 68 stack backtrace. If address is given then it is taken to be the address of the current frame (instead of r4). If count is given then only the first count frames are printed.